1. Field of the Invention
This invention relates to a composite porous bearing with its pores filled with lubricant oil, and a method of making such a bearing. More specifically, the invention relates to a slide bearing supporting a rotating shaft of an axial-flow fan motor or other rotating devices, and a method of making such a bearing.
2. Description of the Related Art
Many ideas are known in the conventional art in relation to a so-called mid-relief bearing where the inside diameter of the middle section is larger than that of either end section. Japanese Patent Laid-open No. 57-110823 and Japanese Patent Laid-open No. 58-94628 relate to a mid-relief bearing manufactured by first removing part of the outer surface of the middle section to make the walls of the middle section thinner and by expanding outwardly the thinner walls from inside to make the inside diameter of the middle section larger during a sizing process. Also, Japanese Patent Publication No. 60-54525 and Japanese Patent Laid-open No. 1-242821 relate to a method of making a mid-relief bearing by sizing a bearing with a special die so that the inside diameter of the middle section gets larger. Japanese Patent Laid-open No. 62-149803 and Japanese Patent Laid-open No. 62-151502 also use similar methods.
Further, Japanese Patent Laid-open No. 2-107705 uses a crowned sizing core during a sizing process and removes it by spring-back. Japanese Utility Model Publication No. 51-53387 and Japanese Patent Laid-open No. 3-240901 make the inside diameter of the middle section larger by means of the pressure applied to the bearing as it is pressed fit into a housing.
All the methods mentioned above obtain a one-piece bearing with a larger inside diameter at the middle section by means of plastic deformation caused by either a special die in a forming process or the press-fit pressure in a housing process, for the purpose of reducing friction found with a long bearing having a relatively large bearing area. These methods have advantage over an earlier three-piece bearing which uses two separate bearing parts and a housing to form a mid-relief bearing assembly. A major advantage is high performance with reduced cost due to the simpler one-piece structure.
Another Japanese Patent Laid-open No. 6-70504 discloses a long-life porous bearing featuring effective use of lubricating oil where the high-load and low-load materials have oil-filled pores of different sizes and are joining together, and oil circulates from the low-load material to the high-load material through their pores by capillary. Another Japanese patent Japanese Utility Model Laid-open No. 2-65714 discloses a long-life porous bearing consisting of two sintered porous materials with pores of different sizes where oil returns into the larger pores in one material and then advances by capillary through the smaller pores in the other material.
Another Japanese Patent Laid-open No. 4-75443 prevents oil leakage by providing the peripheral area of a bearing with oil-collecting grooves. Japanese Utility Model Publication No. 48-39464, Japanese Utility Model Publication No. 44-22002, and Japanese Utility Model Laid-open No. 48-4311 also use similar techniques. In these methods, the grooves in the outside peripheral area allow oil to pass through them, not only preventing oil leakage but improving cooling effect, and an oil-filled felt or other oil supplying means ensures a longer-life bearing.
A mid-relief bearing as mentioned above can reduce bearing friction, but suffers from an accuracy problem in a sizing process because either the middle section is expanded or the end sections are squeezed during a sizing process to make the inside diameter of the middle section larger. Especially, it is practically impossible to control the distribution of pores over the inner peripheral areas because of the one piece manufacturing method. Also it is difficult to make the inside diameters, or the concentric alignment, the shaft-joining end sections accurate. When compared with a three piece bearing assembly where two bearing materials are assembled into a housing, the one piece bearing more quickly deteriorates in performance, requires more lenient dimensional tolerances, has less stable bearing characteristics, and thereby leads to an unsatisfactory product yield.
For a bearing made of different materials tightly joining together and having oil-filled pores, which may be different in size, longer life can be expected due to oil circulation through pores, but tight junction between the materials is assured only by the press-fit pressure at the time of incorporation into a housing and not easily ensured or confirmed. Also, tight junction between the materials is difficult to achieve and may fail to assure effective oil circulation, because the joining areas are sometimes not large enough or have relatively coarser texture than inner or outer peripheral areas. Another problem with this type of bearing is that other sintered materials are necessary outside the bearing to collect and return oil to the bearing, adding to the cost.
While any bearing made by powder-material metallurgy is formed, sintered, sized, and assembled into a housing during manufacturing, some of the conventional art assembles two or more formed materials into one bearing assembly and subjects the assembly to a sintering process ensuring tight junction between materials by making use of the diffusion effect of the process. Some of the conventional art forms, sinters, and sizes bearing materials separately, and joins them together in a press-fit assembly process.
The method in which formed materials are assembled together and then sintered suffers poor workability due to the weak strength of formed materials, as well as limited allowable combination of materials due to the requirements of a sintering process. The method in which materials are sized separately before assembled together requires an additional process of inside diameter correction to make the inside diameters, and the concentric alignment, of the materials accurate and have a desired distribution of pores. Additional process means additional cost.